Support metrics at Source.split for Direct Runner

[Abacn]

Address #32021 for direct runner

• Added a validate runner test for this scenario

Please add a meaningful description for your change here

Tested manually - BUILD SUCCESSFUL for the following commands

./gradlew :runners:direct-java:validatesRunner --tests *testBoundedSourceMetricsInSplit

confirmed the new test already passing on portable runners:

./gradlew :runners:flink:1.18:job-server:validatesPortableRunnerBatch --tests *testBoundedSourceMetricsInSplit

./gradlew :runners:spark:3:job-server:validatesPortableRunnerBatch --tests *testBoundedSourceMetricsInSplit

./gradlew :runners:google-cloud-dataflow-java:validatesRunnerV2Test -PgcpProject=... -PgcpRegion=us-central1 -PgcpTempRoot=gs://.../tmp --tests *testBoundedSourceMetricsInSplit -info

Dataflow legacy runner does not support attempted metrics (and excluded it from validateRunner test). If change to match committed metrics, tested that the test also passes.

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[Abacn]

MetricKey.create has parameter type (@Nullable String, MetricName). It already supported null step

[Abacn]

This will make #31804 testable on Direct runner also -- BQ source lineage were added in BoundedSource.split, currently not testable on Direct runner

[Abacn]

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[Abacn]

R: @Naireen @damondouglas

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[damondouglas]

@Abacn LGTM